Identifying characteristics of a subterranean region using vector-based wavefield separation of seismic data from the subterranean region

Abstract

Methods and systems, including computer programs encoded on a computer storage medium can be used for identifying primary-wave (P-wave) and secondary-wave (S-wave) characteristics of an underground formation by separating P-wave and S-wave modes of seismic data generated by applying a seismic source to a subterranean region of a geological area. Particle motion vectors of a P-wave are parallel to a propagation vector of the P-wave, whereas particle motion vectors of an S-wave are perpendicular to a propagation vector of the S-wave. The parallel and perpendicular relationship between the motion and propagation vectors of the respective P- and S-waves provide a basis for separating P- and S-wave components from a wavefield. The separation methodology extracts P-wave components and S-wave components from the wavefield based on an estimated angle between propagation vectors and wave motion vectors for the wavefield.

What is claimed is: 
     
       1. A computer-implemented method for identifying primary-wave (P-wave) and secondary-wave (S-wave) characteristics of an underground formation by separating P-wave and S-wave modes of seismic data generated by applying a seismic source to a subterranean region of a geological area, the method comprising:
 obtaining a wavefield comprising longitudinal and transverse particle velocity components observed by geophones deployed in the subterranean region; 
 calculating stress components for each of the P-wave and the S-wave modes of seismic data associated with the subterranean region using a wavefield extrapolation engine; 
 computing propagation vectors for the wavefield based on the longitudinal and transverse particle velocity components and the stress components for each of the P-wave and the S-wave modes of seismic data using the wavefield extrapolation engine, 
 wherein computing the propagation vectors comprises computing Poynting vectors of the wavefield based on a first-order elastic wavefield equation that uses the longitudinal or transverse particle velocity components as a first input variable and the P-wave or S-wave mode stress components as a second input variable; 
 estimating an angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield; 
 extracting P-wave components of the wavefield based at least in part on the angle between the propagation vectors and the wave motion vectors using a wavefield separation engine; and 
 extracting S-wave components of the wavefield based at least in part on the angle between the propagation vectors and the wave motion vectors using the wavefield separation engine. 
 
     
     
       2. The method of  claim 1 , wherein obtaining the wavefield comprising longitudinal and transverse particle velocity components comprises:
 applying the seismic source to the subterranean region and detecting a seismic wave that occurs in response to the seismic source being applied to the subterranean region. 
 
     
     
       3. The method of  claim 2 , wherein detecting the seismic wave comprises measuring longitudinal and transverse particle velocity components using one or more geophones in the subterranean region. 
     
     
       4. The method of  claim 1 , further comprising:
 extracting the longitudinal and transverse particle velocity components of the wavefield from information in the seismic data that corresponds to a measured energy flux of the wavefield. 
 
     
     
       5. The method of  claim 1 , wherein calculating stress components for each of the P-wave and the S-wave modes of seismic data comprises:
 calculating the stress components using wavefield extrapolation such that the stress components are calculated without direct manipulation of the wavefield. 
 
     
     
       6. The method of  claim 1 , wherein estimating the angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield comprises:
 estimating the angle in response to generating a least-squares solution for a given time window. 
 
     
     
       7. The method of  claim 6 , wherein extracting the P-wave components of the wavefield comprises:
 extracting the P-wave components of the wavefield using an angle-based weighting function that is applied to the angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield. 
 
     
     
       8. The method of  claim 1 , wherein extracting the S-wave components of the wavefield comprises:
 extracting the S-wave components of the wavefield in response to subtracting the P-wave components of the wavefield. 
 
     
     
       9. A system for identifying primary-wave (P-wave) and secondary-wave (S-wave) characteristics of an underground formation by separating P-wave and S-wave modes of seismic data generated by applying a seismic source to a subterranean region of a geological area, the system comprising:
 a processor; and 
 a non-transitory machine-readable storage device storing instructions that are executable by the processor to cause performance of operations comprising:
 obtaining a wavefield comprising longitudinal and transverse particle velocity components observed by geophones deployed in the subterranean region; 
 calculating stress components for each of the P-wave and the S-wave modes of seismic data associated with the subterranean region using a wavefield extrapolation engine; 
 computing propagation vectors for the wavefield based on the longitudinal and transverse particle velocity components and the stress components for each of the P-wave and the S-wave modes of seismic data using the wavefield extrapolation engine, 
 
 wherein computing the propagation vectors comprises computing Poynting vectors of the wavefield based on a first-order elastic wavefield equation that uses the longitudinal or transverse particle velocity components as a first input variable and the P-wave or S-wave mode stress components as a second input variable;
 estimating an angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield; 
 extracting P-wave components of the wavefield based at least in part on the angle between the propagation vectors and the wave motion vectors using a wavefield separation engine; and 
 extracting S-wave components of the wavefield based at least in part on the angle between the propagation vectors and the wave motion vectors using the wavefield separation engine. 
 
 
     
     
       10. The system of  claim 9 , wherein obtaining the wavefield comprising longitudinal and transverse particle velocity components comprises:
 applying the seismic source to the subterranean region and detecting a seismic wave that occurs in response to the seismic source being applied to the subterranean region. 
 
     
     
       11. The system of  claim 10 , wherein detecting the seismic wave comprises measuring longitudinal and transverse particle velocity components using one or more geophones in the subterranean region. 
     
     
       12. The system of  claim 9 , wherein the operations further comprise:
 extracting the longitudinal and transverse particle velocity components of the wavefield from information in the seismic data that corresponds to a measured energy flux of the wavefield. 
 
     
     
       13. The system of  claim 9 , wherein calculating stress components for each of the P-wave and the S-wave modes of seismic data comprises:
 calculating the stress components using wavefield extrapolation such that the stress components are calculated without direct manipulation of the wavefield. 
 
     
     
       14. The method of  claim 1 , wherein computing propagation vectors for the wavefield comprises:
 computing propagation vectors for the P-wave mode of seismic data based on Poynting vectors of the wavefield that are computed for the P-wave mode; and 
 computing propagation vectors for the S-wave mode of seismic data based on Poynting vectors of the wavefield that are computed for the S-wave mode. 
 
     
     
       15. The method of  claim 1 , wherein the computed Poynting vectors are descriptive of an energy flux of the wavefield from which the longitudinal and transverse particle velocity components are extracted. 
     
     
       16. The system of  claim 9 , wherein estimating the angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield comprises:
 estimating the angle in response to generating a least-squares solution for a given time window. 
 
     
     
       17. The system of  claim 16 , wherein extracting the P-wave components of the wavefield comprises:
 extracting the P-wave components of the wavefield using an angle-based weighting function that is applied to the angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield. 
 
     
     
       18. The system of  claim 9 , wherein extracting the S-wave components of the wavefield comprises:
 extracting the S-wave components of the wavefield in response to subtracting the P-wave components of the wavefield. 
 
     
     
       19. The system of  claim 9 , wherein computing propagation vectors for the wavefield comprises:
 computing propagation vectors for the P-wave mode of seismic data based on Poynting vectors of the wavefield that are computed for the P-wave mode; and 
 computing propagation vectors for the S-wave mode of seismic data based on Poynting vectors of the wavefield that are computed for the S-wave mode. 
 
     
     
       20. The system of  claim 9 , wherein the computed Poynting vectors are descriptive of an energy flux of the wavefield from which the longitudinal and transverse particle velocity components are extracted. 
     
     
       21. A non-transitory machine-readable storage device storing instructions that are executable by a processor to cause performance of operations comprising:
 obtaining a wavefield comprising longitudinal and transverse particle velocity components observed by geophones deployed in a subterranean region; 
 calculating stress components for each of a P-wave mode and an S-wave mode of seismic data associated with the subterranean region using a wavefield extrapolation engine; 
 computing propagation vectors for the wavefield based on the longitudinal and transverse particle velocity components and the stress components for each of the P-wave and the S-wave modes of seismic data using the wavefield extrapolation engine, 
 wherein computing the propagation vectors comprises computing Poynting vectors of the wavefield based on a first-order elastic wavefield equation that uses the longitudinal or transverse particle velocity components as a first input variable and the P-wave or S-wave mode stress components as a second input variable; 
 estimating an angle between the computed propagation vectors for the wavefield and wave motion vectors observed in the wavefield; 
 extracting P-wave components of the wavefield based at least in part on the angle between the propagation vectors and the wave motion vectors using a wavefield separation engine; and 
 extracting S-wave components of the wavefield based at least in part on the angle between the propagation vectors and the wave motion vectors using the wavefield separation engine.